Tissue engineering of heart valves: in vitro experiences

Tissue-Engineered Vessel Strengthens Quickly under Physiological Deformation: Application of a New Perfusion Bioreactor with Machine Vision

In order to develop a patent tissue-engineered blood vessel that grossly resembles native tissue, required culture times in most studies exceed 8 weeks. For the sake of shortening the maturation period of the constructs, we have used deformation as the basic index for mechanical environment control. A new bioreactor with a machine vision identifier was developed to accurately control the deformation of the construct during the perfusion process. Two groups of seeded constructs (n = 4 per group) were investigated in this study, with one group stimulated by a cyclic deformation of 10% and the other by a pulsatile pressure that gradually increased to 120 mm Hg (the control group). After 21 days of culture, the mechanical properties of the constructs were examined. The average burst strength and suture retention strength in the two groups were significantly different (t test, p < 0.05). For the experimental group, the average burst strength and suture retention strength were higher than those of the control group, by 31.6 and 23.4%, respectively. Specifically, the average burst strength of the constructs reached 1,402 mm Hg (close to that of the native vessel, i.e. 1,680 mm Hg) within a relatively short period of 21 days. In conclusion, deformation is an observable, controllable and very valuable index for mechanical environment control in vascular tissue engineering. It makes the control of mechanical stimuli more essential and experiments more comparable.

The application of polyhydroxyalkanoates as tissue engineering materials

Polyhydoxyalkanoates (PHA) are polyesters produced by microorganisms under unbalanced growth conditions. They are generally biodegradable and thermoprocessable, making them attractive as biomaterials for applications in both conventional medical devices and tissue engineering. Over the past years, PHA, particularly poly 3-hydroxybutyrate (PHB), copolymers of 3-hydroxybutyrate and 3-hydroxyvalerate (PHBV), poly 4-hydroxybutyrate (P4HB), copolymers of 3-hydroxybutyrate and 3-hydroxyhexanoate (PHBHHx) and poly 3-hydroxyoctanoate (PHO) and its composites have been used to develop devices including sutures, repair devices, repair patches, slings, cardiovascular patches, orthopedic pins, adhesion barriers, stents, guided tissue repair/regeneration devices, articular cartilage repair devices, nerve guides, tendon repair devices, bone marrow scaffolds, and wound dressings. The changing PHA compositions also allow favorable mechanical properties, biocompatibility, and degradation times within desirable time frames under specific physiological conditions. This paper reviews what have been achieved in the PHA tissue engineering area and concluded that the PHA prospective will look very bright in the near future.

Biomodels of Bone: A Review

In this paper, a definition of a biomodel is presented, based on which different specific types of biomodels are identified, viz., virtual biomodels, computational biomodels, and physical biomodels. The paper then focuses on both physical and virtual biomodels of bone, and presents a review of model generation methodologies, giving examples of typical biomodel applications. The use of macroscale biomodels for such issues as the design and preclinical testing of surgical implants and preoperative planning is discussed. At the microscale, biomodels of trabecular bone are examined and the link with scaffolds for tissue engineering is established. Conclusions are drawn on the state of the art, and the major developments necessary for the continued expansion of the field are identified. Finally, arguments are given on the benefits of integrating the use of the different types of biomodels reviewed in this paper, for the benefit of future research in biomechanics and biomaterials.

Comparison of three myofibroblast cell sources for the tissue engineering of cardiac valves

The objective of this study was to evaluate the capacity of three clinically useful tissue sources: tricuspid valve leaflet (TVL), carotid artery (CA), and jugular vein (JV), to generate myofibroblasts for potential use in a tissue-engineered cardiac valve replacement. Tissue biopsies of clinically appropriate sizes obtained from juvenile sheep were used for this work. Cells obtained from all three tissue sources exhibited a myofibroblast phenotype in vitro, as demonstrated by their immunoreactivity with antibodies directed against vimentin, alpha-smooth muscle actin, fibronectin, and chondroitin sulfate. Protein synthesis characteristics were defined for the key extracellular matrix components: collagen, glycosaminoglycans, and elastin. Among the three sources, JV generated the highest numbers of cells, and JV cells produced the largest amount of collagen per cell. These data suggest that venous tissue, with its relative ease of accessibility, may generate myofibroblasts potentially useful for the interstitial cellular component of a tissue-engineered cardiac valve.

Crosslinked hyaluronan scaffolds as a biologically active carrier for valvular interstitial cells

Hyaluronic acid (HA), a major component of the cardiac jelly during heart morphogenesis, is a polysaccharide that upon modification can be photopolymerized into hydrogels. Previous work in our lab has found that photopolymerizable HA hydrogels are suitable scaffolds for the culture and proliferation of valvular interstitial cells (VICs), the most prevalent cell type in native heart valves. The physical properties of HA gels are easily modified through alteration in material crosslink density or by copolymerizing with other reactive macromolecules. Degradation products of HA gels and the starting macromers significantly increased VIC proliferation when added to cell cultures. With low molecular weight HA (<6700 Da) exhibiting greatest stimulation of VIC proliferation. Low molecular weight HA degradation products added to VIC cultures also resulted in a four-fold increase in total matrix production and a two-fold increase in elastin production over untreated controls. VIC internalization of HA, as shown by cellular uptake of fluorescently labeled HA, likely activates signaling cascades resulting in the biological responses seen here. Lastly, VICs encapsulated within HA hydrogels remained viable, and significant elastin production was observed after 6 weeks of culture. This work shows promise for the creation of a tissue-engineered heart valve utilizing the synergistic relationship between hyaluronic acid and VICs.

Application of tissue-engineering principles toward the development of a semilunar heart valve substitute

Heart valve disease is a significant medical problem worldwide. Current treatment for heart valve disease is heart valve replacement. State of the art replacement heart valves are less than ideal and are associated with significant complications. Using the basic principles of tissue engineering, promising alternatives to current replacement heart valves are being developed. Significant progress has been made in the development of a tissue-engineered semilunar heart valve substitute. Advancements include the development of different potential cell sources and cell-seeding techniques; advancements in matrix and scaffold development and in polymer chemistry fabrication; and the development of a variety of bioreactors, which are biomimetic devices used to modulate the development of tissue-engineered neotissue in vitro through the application of biochemical and biomechanical stimuli. This review addresses the need for a tissue-engineered alternative to the current heart valve replacement options. The basics of heart valve structure and function, heart valve disease, and currently available heart valve replacements are discussed. The last 10 years of investigation into a tissue-engineered heart valve as well as current developments are reviewed. Finally, the early clinical applications of cardiovascular tissue engineering are presented.

Designing scaffolds for valvular interstitial cells: cell adhesion and function on naturally derived materials

Valvular interstitial cells (VICs) possess many properties that make them attractive for use in the construction of a tissue-engineered valve; however, we have found that the surfaces to which VICs will adhere and spread are limited. For example, VICs adhere and spread on collagen and laminin-coated surfaces, but display altered morphology and do not proliferate. Interestingly, fibronectin (FN) was one adhesion protein that facilitated VIC adhesion and proliferation. Yet VICs did not spread on surfaces modified with RGD, a ubiquitous cell-adhesive peptide, nor with other FN-specific peptide sequences such as EILDV and PHSRN. Hyaluronic acid (HA) is a highly elastic polysaccharide that is involved in natural valve morphogenesis and possesses binding interactions with FN. Hyaluronic acid was modified to form photopolymerizable hydrogels, and VICs were found to spread and proliferate on HA-based gels, forming a confluent monolayer on the gels within 4 days. Modified HA retained its ability to specifically bind FN, allowing for the formation of gels containing both HA and FN. Valvular interstital cells cultured on HA surfaces displayed significantly increased production of extracellular matrix proteins, indicating that HA-based scaffolds may provide useful biological cues to stimulate heart valve tissue formation.

Improved Prediction of the Collagen Fiber Architecture in the Aortic Heart Valve

Living tissues show an adaptive response to mechanical loading by changing their internal structure and morphology. Understanding this response is essential for successful tissue engineering of load-bearing structures, such as the aortic valve. In this study, mechanically induced remodeling of the collagen architecture in the aortic valve was investigated. It was hypothesized that, in uniaxially loaded regions, the fibers aligned with the tensile principal stretch direction. For biaxial loading conditions, on the other hand, it was assumed that the collagen fibers aligned with directions situated between the principal stretch directions. This hypothesis has already been applied successfully to study collagen remodeling in arteries. The predicted fiber architecture represented a branching network and resembled the macroscopically visible collagen bundles in the native leaflet. In addition, the complex biaxial mechanical behavior of the native valve could be simulated qualitatively with the predicted fiber directions. The results of the present model might be used to gain further insight into the response of tissue engineered constructs during mechanical conditioning.

Three-Dimensional BioAssembly Tool for Generating Viable Tissue-Engineered Constructs

The primary emphasis of tissue engineering is the design and fabrication of constructs for the replacement of nonfunctional tissue. Because tissue represents a highly organized interplay of cells and extracellular matrix, the fabrication of replacement tissue should mimic this spatial organization. This report details studies evaluating the use of a three-dimensional, direct-write cell deposition system to construct spatially organized viable structures. A direct-write bioassembly system was designed and fabricated to permit layer-by-layer placement of cells and extracellular matrix on a variety of material substrates. Human fibroblasts suspended in polyoxyethylene/polyoxypropylene were coextruded through a positive displacement pen delivery onto a polystyrene slide. After deposition, approximately 60% of the fibroblasts remained viable. Bovine aortic endothelial cells (BAECs) suspended in soluble collagen type I were coextruded via microdispense pen delivery onto the hydrophilic side of flat sheets of polyethylene terephthalate. After deposition with a 25-gauge tip, approximately 86% of the BAECs were viable. When maintained in culture for up to 35 days, the constructs remained viable and maintained their original spatial organization. These results indicate the potential for utilizing a direct-write, three-dimensional bioassembly tool to create viable, patterned tissue-engineered constructs.

Intestinal tissue engineering: from regenerative medicine to model systems

BACKGROUND

Tissue engineering is a rapidly emerging field that combines the established disciplines of engineering, biology, and medicine with the goal of fabricating viable tissues and organs. Although initially targeted for applications in regenerative medicine, a novel application of this technology has been to generate experimental model systems for studying biological mechanisms and testing the efficacy of potential therapies.

METHODS

Review and analysis of the literature.

RESULTS AND CONCLUSION

We present the limitations of existing models and provide examples of how tissue engineering has allowed for the generation of new models that may overcome these limitations.

Bone generation on PHBV matrices: an in vitro study

Bone formation was investigated in vitro by culturing rat marrow stromal osteoblasts in biodegradable, macroporous poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) (PHBV) matrices over a period of 60 days. Foams were prepared after solvent evaporation and solute leaching. PHBV solutions with different concentrations were prepared in chloroform: dichloromethane (1:2, v/v). In order to create a matrix with high porosity and uniform pore sizes, sieved sucrose crystals (300-500 microm) were used. PHBV foams were treated with rf-oxygen plasma (100 W 10 min) to modify their surface chemistry and hydrophilicity with the aim of increasing the reattachment of osteoblasts. Osteoblasts were isolated from rat bone marrow and seeded onto PHBV foams. The cell density on and in the foams was determined with MTS assay. MTS results showed that osteoblasts proliferated on PHBV. Twenty-one days after seeding of incubation, growth of osteoblasts on matrices and initiation of mineralization were observed by confocal laser scanning microscopy. Increasing ALP and osteocalcin secretion during 60 days confirmed the osteoblastic phenotype of the derived stromal cells. SEM, histological evaluations and confocal laser scanning microscopy showed that osteoblasts could grow inside the matrices and lead to mineralization. Cells exhibited spindle-like morphology and had a diameter of 10-30 microm. Based on these, it could confidently be stated that PHBV seems to be a promising polymeric matrix material for bone tissue engineering.

Potential tissue-engineering applications for neonatal surgery

Tissue engineering attempts to build neotissue from its cellular building blocks. This neotissue can then be used for reconstructive surgical applications such as replacement of a congenitally abnormal heart valve or repair of a craniofacial abnormality. Since its inception in the late 1980s, tissue engineering has sparked the interests of physicians and scientists alike because of its great potential. Significant progress has been made in this burgeoning branch of science. This article reviews some of the ongoing preclinical and clinical tissue engineering research as it applies to neonatology.

Decellularization protocols of porcine heart valves differ importantly in efficiency of cell removal and susceptibility of the matrix to recellularization with human vascular cells

OBJECTIVE

We compared 3 different decellularization protocols in porcine heart valves for efficiency of complete cell removal and potential for recellularization.

METHODS

Porcine aortic and pulmonary roots were treated with trypsin, sodium-dodecyl-sulphate, or a new method using 0.25% tert-octylphenyl-polyoxyethylen in combination with sodium-deoxycholate. After a subsequent ribonuclease digestion, specimens were seeded with in vitro expanded human saphenous vein endothelial cells and myofibroblasts.

RESULTS

After treatment with trypsin and subsequent ribonuclease digestion, endothelial attachment took place; however, xenogenic cells were still visible within the matrix. Unexpectedly, when human cells were seeded onto specimens that had been decellularized with sodium-dodecyl-sulphate, the matrices were surrounded by nonviable endothelial cell fragments, indicating a toxic influence of the ionic detergent; 0.25% tert-octylphenyl-polyoxyethylen together with sodium-deoxycholate completely removed porcine cells and enabled host recellularization.

CONCLUSION

Compared with trypsin and sodium-dodecyl-sulphate involving decellularization procedures, reported to be effective in cell removal and susceptible to recellularization with human cells, only the porcine matrix treated with a new detergent-based decellularization method using 0.25% tert-octylphenyl-polyoxyethylen/sodium-deoxycholate followed by nuclease digestion presented an excellent scaffold for recellularization with human cells.

A finite difference model of O2 transport in aortic valve cusps: importance of intrinsic microcirculation

Recent studies have reported the presence of a microcirculation within the tissue of aortic valves. To test the hypothesis that this vascular bed is needed to satisfy the oxygen demands of the cusp tissue, a two-dimensional (2D) finite difference model of oxygen diffusion was developed. The in vivo environment was modeled for vascular and avascular cusps using thickness data from precise radiographic measurements of fresh porcine valves, and O2 diffusivity (DO2) and O2 consumption (VO2) values from experimental data. The location and density of the cusp vasculature were determined by the model to prevent oxygen levels from falling to zero. Validation of the model was performed by simulation of the experimental measurements of cusp DO2 and VO2. For a test cusp with uniform thickness, the model returned simulated DO2 and VO2 measurements within 1.43% and 0.18% difference of the true parameter values, respectively. For native cusps, the simulated DO2 measurements were sensitive to thickness variations (-38 to +21% difference), whereas the VO2 measurements were minimally affected (8% difference). An improved DO2 measurement technique was found to reduce these errors to <5% and is recommended for analysis of experimental data. In the avascular case, the model predicted large regions of hypoxic tissue, whereas in the vascular case, the model predicted vessel locations and densities similar to what was experimentally observed in porcine cusps. Overall, the in vivo model developed in this study confirmed the need for an intrinsic microcirculation in the thicker basal regions of aortic cusps.

Heart valve and arterial tissue engineering

In the industrialized world, cardiovascular disease alone is responsible for almost half of all deaths. Many of the conditions can be treated successfully with surgery, often using transplantation techniques; however, autologous vessels or human-donated organs are in short supply. Tissue engineering aims to create specific, matching grafts by growing cells on appropriate matrices, but there are many steps between the research laboratory and the operating theatre. Neo-tissues must be effective, durable, non-thrombogenic and non-immunogenic. Scaffolds should be bio-compatible, porous (to allow cell/cell communication) and amenable to surgery. In the early days of cardiovascular tissue engineering, autologous or allogenic cells were grown on inert matrices, but patency and thrombogenicity of grafts were disappointing. The current ethos is toward appropriate cell types grown in (most often) a polymeric matrix that degrades at a rate compatible with the cells' production of their own extracellular matrical proteins, thus gradually replacing the graft with a living counterpart. The geometry is crucial. Computer models have been made of valves, and these are used as three-dimensional patterns for mass-production of implant scaffolds. Vessel walls have integral connective tissue architecture, and application of physiological level mechanical forces conditions bio-engineered components to align in precise orientation. This article reviews the concepts involved and successes achieved to date.

Computational analyses of mechanically induced collagen fiber remodeling in the aortic heart valve

To optimize the mechanical properties and integrity of tissue-engineered aortic heart valves, it is necessary to gain insight into the effects of mechanical stimuli on the mechanical behavior of the tissue using mathematical models. In this study, a finite-element (FE) model is presented to relate changes in collagen fiber content and orientation to the mechanical loading condition within the engineered construct. We hypothesized that collagen fibers aligned with principal strain directions and that collagen content increased with the fiber stretch. The results indicate that the computed preferred fiber directions run from commissure to commissure and show a strong resemblance to experimental data from native aortic heart valves.

Comparison of different decellularization procedures of porcine heart valves

BACKGROUND

Tissue engineering of heart valves should avoid the disadvantages of conventional prostheses. In this study we tested different decellularization procedures for their potential of cell removal and their ability to preserve the matrix.

METHODS

Specimens of porcine aortic and pulmonary roots were treated with either trypsin or sodium-dodecyl-sulfate (SDS) or Triton-X 100 and sodium-deoxycholate with a range of concentrations. Tissue samples were then processed for scanning electron microscopy and laser scanning microscopy.

RESULTS

Trypsin achieved only incomplete decellularization and caused severe structural alterations of the matrix. In contrast SDS removed cells completely but caused strong structural alterations. Treatment with Triton-X100 and sodium-deoxycholate achieved both complete decellularization and preservation of the matrix structure.

CONCLUSION

Techniques of decellularization are highly variable in efficiency and matrix preservation and was best achieved in our study with Triton-X100 and sodium deoxycholate.

Cell characterization of porcine aortic valve and decellularized leaflets repopulated with aortic valve interstitial cells: the VESALIO Project (Vitalitate Exornatum Succedaneum Aorticum Labore Ingenioso Obtenibitur)

BACKGROUND

Heart valve bioprostheses for cardiac valve replacement are fabricated by xeno- or allograft tissues. Decellularization techniques and tissue engineering technologies applied to these tissues might contribute to the reduction in risk of calcification and immune response. Surprisingly, there are few data on the cell phenotypes obtained after cellularizing these naturally-derived biomaterials in comparison to those expressed in the intact valve.

METHODS

Aortic valve interstitial cells (VIC) were used to repopulate the corresponding valve leaflets after a novel decellularization procedure based on the use of ionic and nonionic detergents. VIC from leaflet microexplants at the third passage were utilized to repopulate the decellularized leaflets. Intact, decellularized and repopulated valve leaflets and cultured VIC were examined by immunocytochemical procedures with a panel of antibodies to smooth muscle and nonmuscle differentiation antigens. Intact and cellularized leaflets were also investigated with Western blotting and transmission electron microscopy, respectively.

RESULTS

Myofibroblasts and smooth muscle cells (SMC) were mostly localized to the ventricularis of the leaflet whereas fibroblasts were dispersed unevenly. Cultured VIC were comprised of myofibroblasts and fibroblasts with no evidence of endothelial cells and SMC. Two weeks after VIC seeding into decellularized leaflets, grafted cells were found penetrating the bioscaffold. The immunophenotypic and ultrastructural properties of the grafted cells indicated that a VIC heterogeneous mesenchymal cell population was present: fibroblasts, myofibroblasts, SMC, and endothelial cells.

CONCLUSIONS

VIC seeding on detergent-treated valve bioscaffolds has the cellular potential to reconstruct a viable aortic valve.

Tissue engineering

Tissue engineering will potentially change the practice of plastic surgery more than any other clinical specialty. It is an interdisciplinary field that promises new methods of tissue repair. There has been more than $3.5 billion invested in this field since 1990. Relevant areas of progress include advanced computing, biomaterials, cell technology, growth factor fabrication and delivery, and gene manipulation. Beneficial clinical techniques will emerge from continued investigation in each of these areas. Techniques that are developed must be scaled up to industry with products cleared by regulatory agencies and acceptable to clinicians and patients. A goal of tissue engineering is to change clinical practice, yielding improved patient outcomes and lower costs of care.

Tissue-engineering bioreactors: a new combined cell-seeding and perfusion system for vascular tissue engineering

One approach to the tissue engineering of vascular structures is to develop in vitro conditions in order ultimately to fabricate functional vascular tissues before final implantation. In our experiment, we aimed to develop a new combined cell seeding and perfusion system that provides sterile conditions during cell seeding and biomechanical stimuli in order to fabricate autologous human vascular tissue in vitro. The cell seeding and perfusion system is made of Plexiglas and is completely transparent (Berlin Heart, Berlin, Germany; University Hospital Benjamin Franklin, Berlin, Germany). The whole system consists of a cell seeding chamber that can be incorporated into the perfusion system and an air-driven respirator pump connected to the bioreactor. The cell culture medium continuously circulates through a closed-loop system. We thus developed a cell seeding device for static and dynamic seeding of vascular cells onto a polymeric vascular scaffold and a closed-loop perfused bioreactor for long-term vascular conditioning. The cell seeding chamber can be easily connected to the bioreactor, which combines continuous, pulsatile perfusion and mechanical stimulation to the tissue-engineered conduit. Adjusting the stroke volume, the stroke rate, and the inspiration/expiration time of the ventilator allows various pulsatile flows and different levels of pressure. The whole system is a highly isolated cell culture setting, which provides a high level of sterility and a gas supply and fits into a standard humidified incubator. The device can be sterilized by ethylene oxide and assembled with a standard screwdriver. Our newly developed combination of a cell seeding and conditioning device provides sterile conditions and biodynamic stimuli for controlled tissue development and in vitro conditioning of an autologous tissue-engineered vessel.

Use of autologous auricular chondrocytes for lining artificial surfaces: a feasibility study

BACKGROUND

Auricular elastic cartilage is a potential source of autologous cells for lining the luminal surfaces of cardiovascular prostheses. We tested this potential in vitro and in vivo using a left ventricular assist device (LVAD) and a calf model.

METHODS

In vitro, auricular cartilage was harvested from the anesthetized ear of a calf, isolated, and cultured on tissue culture dishes. Primary chondrocytes were typed by immunocytochemistry, transferred into culture media, passaged twice, and seeded onto the blood-contacting luminal surfaces of four LVADs (HeartMate; Thoratec Corporation, Woburn, MA). Seeded cell linings were preconditioned under simulated flow conditions to promote cell adhesion to luminal surfaces. Seeding efficiency and cumulative cell loss under flow conditions were quantitated. In vivo, one of the four autologous chondrocyte-lined and preconditioned LVADs was implanted into the tissue-donor calf; run for 7 days; explanted; and evaluated grossly, by scanning electron microscopy, and by transmission electron microscopy.

RESULTS

The efficiency of seeding chondrocytes onto the luminal surfaces of the four LVADs was 95.11% +/- 4.23% (n = 4). Cumulative cell loss during preconditioning under flow conditions in vitro did not exceed 12% (n = 4). After 7 days of in vivo implantation, the luminal surfaces of the implanted LVAD demonstrated an intact, strongly adherent cellular lining.

CONCLUSIONS

Auricular elastic cartilage is a ready and easily accessible source of chondrocytes whose ability to produce collagen II and other important extracellular matrix constituents allows them to adhere strongly to the luminal surfaces of LVADs. The simple method of isolating and expanding auricular chondrocytes presented here could be used to provide strongly adherent autologous cell linings for LVADs and other cardiovascular devices. If and when chondrocytes can be genetically engineered to produce antithrombogenic factors and then used to line the luminal surfaces of LVADs or other cardiovascular prostheses, they may be able to improve the hemocompatibility of the blood-biomaterial interface in such devices. Our successful feasibility study in a calf model warrants further studies of this concept in vivo.

Low pressure plasma treatment of poly(3-hydroxybutyrate): toward tailored polymer surfaces for tissue engineering scaffolds

Thin films of poly(3-hydroxybutyrate) were modified by microwave ammonia plasma treatment. The results of the modification were studied by means of contact angle goniometry, ellipsometry, Fourier transform infrared-attenuated total reflection spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. To prove the presence of amino groups on the poly(3-hydroxybutyrate) surface, chemical labeling with 4-trifluoromethyl benzaldehyde was performed before X-ray photoelectron spectroscopy analysis. Under the applied plasma conditions, a hydrophilic surface with a good long-term stability was obtained.

Involvement of glnB, glnZ, and glnD genes in the regulation of poly-3-hydroxybutyrate biosynthesis by ammonia in Azospirillum brasilense Sp7

The role of three key nitrogen regulatory genes, glnB (encoding the P(II) protein), glnZ (encoding the P(z) protein), and glnD (encoding the GlnD protein), in regulation of poly-3-hydroxybutyrate (PHB) biosynthesis by ammonia in Azospirillum brasilense Sp7 was investigated. It was observed that glnB glnZ and glnD mutants produce substantially higher amounts of PHB than the wild type produces during the active growth phase. glnB and glnZ mutants have PHB production phenotypes similar to that of the wild type. Our results indicate that the P(II)-P(z) system is apparently involved in nitrogen-dependent regulation of PHB biosynthesis in A. brasilense Sp7.

New pulsatile bioreactor for fabrication of tissue-engineered patches

To date, one approach to tissue engineering has been to develop in vitro conditions to ultimately fabricate functional cardiovascular structures prior to final implantation. In our current experiment, we developed a new pulsatile flow system that provides biochemical and biomechanical signals to regulate autologous patch-tissue development in vitro. The newly developed patch bioreactor is made of Plexiglas and is completely transparent (Mediport Kardiotechnik, Berlin). The bioreactor is connected to an air-driven respirator pump, and the cell culture medium continuously circulates through a closed-loop system. We thus developed a closed-loop, perfused bioreactor for long-term patch-tissue conditioning, which combines continuous, pulsatile perfusion and mechanical stimulation by periodically stretching the tissue-engineered patch constructs. By adjusting the stroke volume, the stroke rate, and the inspiration/expiration time of the ventilator, it allows various pulsatile flows and different levels of pressure. The whole system is a highly isolated cell culture setting, which provides a high level of sterility, gas supply, and fits into a standard humidified incubator. The bioreactor can be sterilized by ethylene oxide and assembled with a standard screwdriver. Our newly developed bioreactor provides optimal biomechanical and biodynamical stimuli for controlled tissue development and in vitro conditioning of an autologous tissue-engineered patch.

Application of stereolithography for scaffold fabrication for tissue engineered heart valves

A crucial factor in tissue engineering of heart valves is the functional and physiologic scaffold design. In our current experiment, we describe a new fabrication technique for heart valve scaffolds, derived from x-ray computed tomography data linked to the rapid prototyping technique of stereolithography. To recreate the complex anatomic structure of a human pulmonary and aortic homograft, we have used stereolithographic models derived from x-ray computed tomography and specific software (CP, Aachen, Germany). These stereolithographic models were used to generate biocompatible and biodegradable heart valve scaffolds by a thermal processing technique. The scaffold forming polymer was a thermoplastic elastomer, a poly-4-hydroxybutyrate (P4HB) and a polyhydroxyoctanoate (PHOH) (Tepha, Inc., Cambridge, MA). We fabricated one human aortic root scaffold and one pulmonary heart valve scaffold. Analysis of the heart valve included functional testing in a pulsatile bioreactor under subphysiological and supraphysiological flow and pressure conditions. Using stereolithography, we were able to fabricate plastic models with accurate anatomy of a human valvular homograft. Moreover, we fabricated heart valve scaffolds with a physiologic valve design, which included the sinus of Valsalva, and that resembled our reconstructed aortic root and pulmonary valve. One advantage of P4HB and PHOH was the ability to mold a complete trileaflet heart valve scaffold from a stereolithographic model without the need for suturing. The heart valves were tested in a pulsatile bioreactor, and it was noted that the leaflets opened and closed synchronously under subphysiological and supraphysiological flow conditions. Our preliminary results suggest that the reproduction of complex anatomic structures by rapid prototyping techniques may be useful to fabricate custom made polymeric scaffolds for the tissue engineering of heart valves.

Cardiovascular tissue engineering: a new laminar flow chamber for in vitro improvement of mechanical tissue properties

A new in vitro flow system was developed to investigate the impact of laminar flow on extracellular matrix formation and tissue development. The dynamic in vitro system was designed to provide a cross flow arrangement of main flow induced by a dialysis roller pump (500 ml/min), and nutrition flow by a perfusion pump (3 ml/hr). Poly-L-lysine precoated polyglycolic acid (PGA) scaffolds (3.14 cm2) were seeded with myofibroblasts of human aortic origin (3.0 x 10(6) cells/ mesh) and incubated for 14 days under static conditions. The tissue was exposed to shear stress over a time period of 14 days (n = 4). The control group was seeded under static conditions (n = 4). To obtain a CO2 independent medium, 25 mM HEPES and 1 mM bicarbonate buffer was supplemented to modified MEM without bicarbonate. Gas samples were collected from the medium, and hydroxyproline assay was performed as a marker of collagen production. The newly developed flow system maintained stable cell culture conditions, with the hydroxyproline concentration significantly higher in group F (p < 0.05). These preliminary experiences with a new in vitro tissue culture system demonstrate the feasibility of using flow induced mechanical stress to enhance extracellular matrix formation.

Oxygen diffusion and consumption of aortic valve cusps

To maintain tissue oxygenation, normal aortic valves contain a vascular bed where tissue thickness is greatest. Avascular "living" tissue-engineered heart valves have been proposed, yet little information exists regarding the magnitude of valve tissue metabolic activity or oxygen requirements. We therefore set out to measure the oxygen diffusivity (DO(2)) and oxygen consumption (VO(2)) of seven porcine aortic valve cusps in vitro at 37 degrees C using a chamber with a Clark oxygen sensor. Mean DO(2) and VO(2) were 1.06 x 10(-5) cm(2)/s and 3.05 x 10(-5) x ml O(2). ml tissue(-1) x s(-1), respectively. When modeled as a three-layered structure by using these values and a boundary condition of 100 mmHg at both surfaces, the average aortic cusp predicted a central mean PO(2) of 27 mmHg (range of 0-50 mmHg). The DO(2) value obtained was similar to that found for other vascular structures, but because our studies were carried out in vitro, the VO(2) measurements may be lower than that required by the functioning valves. These values provide an initial understanding of the oxygen supply possible from the cusp surfaces and the oxygen needs of the tissue.

Advances in congenital heart surgery

Congenital heart surgery is a young and constantly evolving field. Since the first patent ductus arteriosus ligation by Robert Gross of Boston in 1938, a greater understanding of the anatomy and pathophysiology of congenital heart disease, improved diagnostics, and the advent of cardiopulmonary bypass and deep hypothermic circulatory arrest have allowed the open repair of many lesions. Further advances in preoperative, intraoperative, and postoperative patient management have resulted in greatly improved survivals for even the most complex congenital defects. By looking forward through continual technical innovation and back with the critical evaluation of established techniques, we continue to advance the care of the patient with congenital heart disease.